Phase I: oxidation by CYP450
Phase I biotransformation is carried out mainly by the cytochrome P450 (CYP450) family of enzymes, concentrated in the liver. These enzymes oxidize, reduce, or hydrolyze fat-soluble compounds, typically adding or exposing a reactive functional group. The aim is to make a molecule chemically reactive enough for the next step, but the intermediates produced can themselves be more reactive or more toxic than the original substance.
Many drugs and environmental chemicals are processed here, which is also why CYP450 activity is central to drug interactions: one substance can speed up or slow down the metabolism of another.
Phase II: conjugation and excretion
Phase II enzymes attach a polar molecule to the Phase I intermediate, a process called conjugation. Common pathways include glucuronidation, sulfation, glutathione conjugation, acetylation, and methylation. Adding these water-soluble groups neutralizes reactive intermediates and makes the compound easier to excrete in bile or urine.
The two phases are meant to work in balance. If Phase I outpaces Phase II, reactive intermediates can accumulate, which is one reason the simple word detoxification understates the chemistry involved.
A note on context
This describes normal liver biochemistry and is not medical advice. Claims that specific foods, cleanses, or supplements meaningfully boost these pathways are not well supported, and anyone with liver concerns or on medication should consult a qualified clinician.